Booting DT kernel on a non-DT bootloader

[Simon Horman]

Hi,

it has come to pass that for some of the boards under mach-shmobile
we are in the following situation:

* The kernel requires a device tree for booting
* The default boot loader for the board does not support device tree;
  and in general it is not practical for developers to upload the
  boot-loader.

Fortunately there is a solution, which is to append a device tree to
the kernel image and boot that by the normal means supported by the
boot loader.

Here is how to achieve that:

1. Make sure you have CONFIG_ARM_APPENDED_DTB=y in your .config

2. Create original zImage

    ARCH=arm make zImage

3. Create flattened device tree (run this in kernel tree)

   i) There is code in the next branch of my renesas tree to allow
      this to be achieved using

      # make dtbs

      This will create the device tree for the prevaling kernel config,
      if one exists.

      e.g For Armadillo 800 EVA

      # make dtbs
      scripts/kconfig/conf --silentoldconfig Kconfig
        DTC     arch/arm/boot/r8a7740-armadillo800eva.dtb
      DTC: dts->dtb  on file "arch/arm/boot/dts/r8a7740-armadillo800eva.dts"

      Else it will do nothing
      # make dtbs
      scripts/kconfig/conf --silentoldconfig Kconfig
      make[1]: Nothing to be done for `arch/arm/boot/dtbs'.


   ii) If you are running a current or other kernel, which does not
       support "make dtbs" for mach-shmobile boards, then the dtb
       may manually be compiled using dtc.

       e.g. For the Armadillo 800 EVA

       dtc -I dts -O dtb arch/arm/boot/dts/r8a7740-armadillo800eva.dts \
           -o arch/arm/boot/dts/r8a7740-armadillo800eva.dtb

    Note: Please make sure you have a recent version of dtc.
          I have verified that the above works with DTC 1.3.0 from
	  the Debian device-tree-compiler 1.3.0-2 package.
	  I have had reports that it fails using DTC 1.2.0.

4. Append device tree to zImage (very high-tech!)

   cat arch/arm/boot/zImage \
	arch/arm/boot/dts/r8a7740-armadillo800eva.dtb >
	arch/arm/boot/zImage.dtb

5. Create uImage (optional)

   If you need to create a uImage then this may be done using
   the new zImage + dtb as follows:

   # cp arch/arm/boot/zImage.dtb arch/arm/boot/zImage
   # $(cut -f 3- -d ' ' < arch/arm/boot/.uImage.cmd)

6. You should now be able to either boot the resulting zImage.dtb
   or uImage, use whichever suits the needs of the bootloader in use.